The ALPINE-ALMA [C II] Survey: [C II]158micron Emission Line Luminosity Functions at $z \sim 4-6$

We present the [CII]158μm line luminosity functions (LFs) at z ∼ 4−6 using the ALMA observations of 118 sources, which are selected to have UV luminosity M_(1500A) 10⁹ L_⊙) compared to z∼0. This excess is consistent with a potential population of [C II]-bright UV-faint sources that are largely excluded from the ALPINE target selection. To further constrain such sources, in a companion analysis, Loiacono et al. (2020) estimate the [C II] LF based on serendipitous [C II] sources in the ALPINE maps. Their resulting single data point is consistent with the excess we observe at high-L. Combined, these studies suggest possible redshift evolution in the [C II] LFs at high luminosity between z ∼ 5 and 0. This conclusion is further supported by the inferred [C II] LFs from the far-IR and CO LFs at z ∼ 4−6. Integrating our estimate on the best-fit [C II] LF, we set a constraint on molecular gas mass density at z ∼ 4−6, with ρ_(mol) ∼ (2−7)×10⁷ M_⊙ Mpc⁻³. This is broadly consistent with previous studies, although the uncertainties span more than an order of magnitude. Finally, we find that available model predictions tend to underestimate the number densities of [C II] emitters at z ∼ 5.